A storage of energy is possible according to the “power to gas” concept by creating storable methane. First of all, hydrogen is generated with unused peaks of regeneratively produced energy by electrolysis. From hydrogen (H2) together with carbon dioxide (CO2) storable methane (CH4) can be produced. The methane formation can be written by the following equation: 4H2+CO2⇔CH4+2H2O (so-called Sabatier reaction). This reaction can be carried out in a purely chemical and in a biological way. The chemical reaction is constrained at temperatures above 180° C. and high pressures and is make possible by relatively demanding catalysts.
The biological transformation occurs by special microorganisms. In natural, anaerobic, aqueous ecosystems such as marshes, sewage sludge or flooded soil, consortia of several groups of organisms form methane through a chain of decomposition from organic material such as plant and animal remains. There are mesophilic consortia and thermophilic consortia. Their temperature optima lie at 30-40° C. and 50-60° C., respectively. The methane bacteria form the last link in this chain of decomposition, which break down organic substrate through many intermediate steps such as organic acids, alcohols and hydrogen, into methane and carbon dioxide. Methane bacteria are among the Archaebacteria (Archaea). They live only under oxygen-free conditions, have very specific nutrient requirements, grow very slowly and have a very restricted substrate spectrum. Some methane bacteria utilize acetic acid or formic acid to form methane. Other methane bacteria form methane by the above equation from hydrogen and carbon dioxide.
The known technical layouts for biogenic methane formation are not suited to the conversion of gaseous substrates. Such layouts as biogas plants, digestion towers, or plants for anaerobic sludge treatment have been optimized exclusively to biomethanize hydrolyzable or dissolved organic substrates via the mentioned chains of decomposition. Hydrogen is also formed as an intermediate product. It needs a very low hydrogen partial pressure of less than 10−4 bar in order to further break down the resulting organic intermediate products. Hydrogen consuming methane bacteria maintain the necessary low hydrogen partial pressure. If hydrogen were added to this process, the entire decomposition chain could be damaged and brought to a standstill.
If methane is to be produced with the above described hydrogen-utilizing bacteria, the highest possible hydrogen partial pressure should prevail, unlike the case of the above chain of decomposition, in order to supply the bacteria with their gaseous substrate. The groups of organisms upstream from the methane bacteria in the chain of decomposition then no longer play any role. Thus, more gas should be supplied than the bacteria are able to convert, in order to achieve a maximum conversion rate.
Pure cultures of such organisms are supplied with their substrates directly via the gas phase for research purposes on the laboratory scale (reagent glasses, flasks, etc.). The organisms draw in the gases metabolized by them from the gas phase by a gradient and give off the methane formed to the gas phase. Such pure cultures are not used on an industrial scale, since being strict anaerobes they are killed by even traces of oxygen. Thus far, neither was there any incentive to use them. The organisms need to be supplied with gas in a different way on an industrial scale.
Given this background, and as part of new energy storage concepts (power to gas), methods have been proposed to use methane bacteria for conversion of hydrogen and carbon dioxide that are designed to increase the gas exchange surface: solid bed reactors with large gas phase. Suitable organisms grow here as biofilms on substrate materials, being flushed with nutritive medium and supplied from the gas phase. The drawback of these methods is that these surfaces first need to be overgrown by the organisms. But methane bacteria grow very slowly and even under optimal conditions they have generation times of several days. Furthermore, it is questionable how methane bacteria could even form a biofilm. The startup phase of such reactors is very long in practice, uncertain, and hardly controllable. If the biofilms later become too thick, the organisms in the interior can only be supplied suboptimally and “dead zones” will result. The biomass concentration per reactor volume can thus only be controlled with difficulty. After an injury to the organisms by a malfunction, such reactors can only slowly be placed back in operation. There is no known use of these methods in practice.
In summary, it can be said that the generation and storage of SNG (Substitute Natural Gas) in the natural gas grid represents a highly promising option for storing energy from renewable sources by making use of the existing natural gas infrastructure and operating with high efficiencies. But there is still a need to optimize the methane gas production.
The problem which the present invention proposes to solve is therefore to provide effective and economical means and methods with which methane can be produced by making use of methanogenic microorganisms.